1. Field of the Invention
This invention relates to a magneto-optical recording medium.
2. Prior Art
Optical discs are of great interest as high capacity information carrying media. Among the optical discs, magneto-optical discs of the magnetic field modulation system are expected to find use in data files or the like. A magneto-optical disc is recorded in a magnetic field modulation mode by directing a laser beam from an optical head to the recording layer of the disc in a DC manner for raising the temperature thereof and simultaneously applying a modulated magnetic field across the recording layer from a magnetic head opposed to the optical head. Therefore, the magnetic field modulation mode allows for overwrite recording.
Recently, a magneto-optical disc of the magnetic field modulation mode (known as a mini-disc) which can record and reproduce signals at low linear velocity equivalent to the linear velocity (1.2 m/s to 1.4 m/s) of compact discs (CDs) has been put into commercial practice. This magneto-optical disc can share a drive unit with read-only optical discs of the specification complying with the CD. The potential applications of the drive unit for this magneto-optical disc include a portable type where it is required to reduce power consumption. However, the magneto-optical disc of the magnetic field modulation mode needs a recording magnetic head as well as an optical head, often leading to an increased power consumption. Therefore, there is a desire for a disc which is so sensitive to a magnetic field as to enable recording in a low magnetic field. The low magnetic field recording enables power consumption and hence battery size to be reduced. There is also obtained an advantage in designing of the magnetic head in that the number of coil windings in the magnetic head can be reduced.
However, conventional magneto-optical discs, for example, 3.5-inch magneto-optical discs complying with the ISO standard generally require a recording magnetic field of 200 Oe or higher in order to obtain sufficient C/N. For the above-mentioned mini-discs, on the other hand, it is desired to reduce the recording magnetic field strength to below 100 Oe, preferably below 80 Oe in order to minimize power consumption. It is also desired that at least 46 dB C/N be achieved at such a low magnetic field strength.
In order to use them for data discs for computer purposes, it is required to increase the recording linear velocity for achieving an increased data transfer speed. CD-ROMs have now been used at double, treble and quadruple the linear velocity. In the mini-discs, too, it is required to use a linear velocity at least twice as high. High linear velocity results in a C/N drop even at the same recording magnetic field strength; that is, for recording at high linear velocity it is required to prevent the C/N drop by increasing the magnetic field strength. With conventional magneto-optical discs highly sensitive to magnetic fields, however, it is impossible to obtain sufficient C/N even when the magnetic field is increased corresponding to the high linear velocity. In other words, when the magnetic field sensitivity is increased, there is some considerable decrease in the peak C/N value. Never until now is any magneto-optical disc enabling high C/N to be achieved in both low and high magnetic fields achieved.
Japanese Patent Application Kokai (JP-A) No. 313835/1992 proposes "a method for preparing a magneto-optical recording medium by successively forming at least a rare earth element-transition metal alloy thin film and a dielectric film on a substrate wherein after the rare earth element-transition metal alloy thin film is formed, the substrate is maintained in an as-discharged atmosphere, the atmosphere containing at least oxygen gas or a gas of a compound containing oxygen as one constituent element", and alleges that high C/N is accomplished in a low recording magnetic field. However, rare earth elements are readily oxidizable materials as is well known in the art. Indeed, experimentation made by the inventors has revealed that it is very difficult to place the amount of the gas of a compound containing oxygen as one constituent element (CO.sub.2, etc.) under optimum control and it is difficult to achieve the desired properties in a consistent manner. The incorporation of oxygen in the magnetic layer formed of such a rare earth element-transition metal alloy is found to offer another problem in that the magnetic field sensitivity drops gradually upon accelerated testing at high-temperature and high humidity.
Besides, Ichitani et al., "Recording field sensitivity of magneto-optical disks using very thin exchange-coupled film", J. Magn. Soc. Jpn., Vol. 17, Supplement No. S1 (1993), pp. 196-200 describe that a high magnetic field sensitivity suitable for the magnetic field modulation mode is achievable by using a laminate film of GdFeCo and TbFeCo as the recording film. With this method, however, difficulty is involved in cost reductions due to an increase in the number of steps.